1. Field of the Invention
The present invention relates to a photomask used for forming a fine pattern in fabrication of a semiconductor integrated circuit device and a pattern formation method using the photomask.
2. Description of Related Art
Recently, there are increasing demands for refinement of a circuit pattern due to a high degree of integration of a large scale integrated circuit device (hereinafter referred to as an LSI) realized by using a semiconductor. As a result, great importance is placed on refinement of a wiring pattern forming the circuit or a contact hole pattern for connecting multiple wiring layers with insulating layers interposed therebetween (hereinafter referred to as a contact pattern). For the formation of the contact pattern, a technique of simultaneously forming isolated patterns and densely arranged patterns has been required. For achieving large depth of focus in the formation of the densely arranged contact patterns, for example, it is effective to perform oblique-incidence exposure (off-axis exposure) just like for the formation of the densely arranged contact patterns. Specifically, the oblique-incidence exposure is essential for the formation of the dense contact patterns. However, the oblique-incidence exposure considerably deteriorates contrast and the depth of focus of the isolated contact patterns. The deterioration of the contrast and the depth of focus occurs more significantly when a half-tone phase-shifting mask, which is widely used for improvement in resolution, is used. On the other hand, when a light source of low coherence suitable for the formation of isolated fine contact patterns is used, the formation of the dense patterns becomes difficult.
As described above, the optimum lighting condition for the isolated fine contact patterns is contradictory to the optimum lighting condition for the densely arranged contact patterns. Therefore, in order to form the densely arranged fine patterns and the isolated patterns at the same time, tradeoff is made on the effect of a vertical incident light component and the effect of an oblique incident light component emitted from a light source. As a result, a light source having a medium degree of coherence (about 0.5 to 0.6) is used. In this case, however, the effects of the vertical incident light component and the oblique incident light components cancel each other. This makes it difficult to achieve the simultaneous refinement of the dense patterns and the isolated patterns to enhance the degree of integration of the semiconductor device. One of effective solutions to this problem is use of an enhancer mask.
Hereinafter, the principle of pattern formation using the enhancer mask is explained with reference to FIGS. 13A to 13D.
FIGS. 13A and 13B show an exemplified plane structure and an exemplified cross-sectional structure of the enhancer mask. As shown in FIGS. 13A and 13B, the enhancer mask includes a transparent substrate 1 having a transparent property against exposing light, on which provided are a semi-light-shielding portion 2 having a predetermined transmittance to the exposing light, an opening 3 surrounded by the semi-light-shielding portion 2 and having a transparent property against the exposing light and a phase-shifting portion 4 surrounded by the semi-light-shielding portion 2 and located on the periphery of the opening 3. The semi-light-shielding portion 2 and the opening 3 transmit the exposing light in an identical phase. The phase-shifting portion 4 transmits the exposing light in an opposite phase with respect to the semi-light-shielding portion 2 and the opening 3, but is not transferred on a wafer by exposure. The phase-shifting portion 4 is provided by, for example, forming an opening in the semi-light-shielding portion 2 and digging an exposed part of the transparent substrate 1 by a predetermined depth.
FIG. 13C is a diagram for showing a light intensity profile obtained by using the enhancer mask of FIGS. 13A and 13B, and FIG. 13D is a schematic diagram for showing the plane structure of a pattern to be transferred onto a wafer when the opening 3 of FIGS. 13A and 13B is a hole pattern. As shown in FIG. 13C, in the photomask shown in FIGS. 13A and 13B, light passing through the phase-shifting portion 4 on the periphery of the opening 3 cancels a portion of lights passing through the opening 3 and the semi-light-shielding portion 2, respectively. Therefore, if the intensity of light passing through the phase-shifting portion 4 is adjusted so that the light passing around the opening 3 is canceled, a dark portion in which the light intensity corresponding to the light passing around the opening 3 is reduced to approximately 0 is formed in the light intensity distribution. The light passing through the phase-shifting portion 4 pronouncedly cancels the light passing around the opening 3, while it slightly cancels light passing near the center of the opening 3. As a result, the light intensity profile of the light passing through the enhancer mask becomes steep in part thereof from the center of the opening 3 to the periphery of the opening 3. Thus, the light passing through the enhancer mask shows a sharp light intensity profile. Therefore, as shown in FIG. 13D, a high contrast image is formed.
As described above, with the provision of the phase-shifting portion 4 along the outline of the opening 3 on the mask with the low-transmittance semi-light-shielding portion 2 on the transparent substrate 1, a considerably dark portion corresponding to the outline of the opening 3 is formed in a light intensity image formed by using the mask (enhancer mask). This makes it possible to provide light intensity distribution with enhanced contrast between the light intensity of the opening 3 and the light intensity around the opening 3. Thus, the enhancer mask is highly effective for the simultaneous formation of the fine isolated contact pattern (e.g., each side of the opening 3 is smaller than (0.5×λ/NA) (λ: a wavelength of the exposing light, NA: numerical aperture)) and the densely arranged contact patterns.
The enhancer mask is actually effective for the above-described fine pattern formation. However, use of the enhancer mask brings about the following problem for the formation of relatively large contact patterns (e.g., each side of the opening is larger than (0.5×λ/NA) (λ: a wavelength of the exposing light, NA: numerical aperture)) typified by accessory patterns, such as a reticle position monitor pattern for aligning a reticle with an aligner, an overlay monitor pattern for overlaying an upper layer on a lower layer and other patterns.
FIGS. 14A and 14B show an exemplified plane structure and an exemplified cross-sectional structure of the enhancer mask. In FIGS. 14A and 14B, like reference numerals are used to refer to like elements of the photomask of FIGS. 13A and 13B so as to avoid repetition of the description. The enhancer mask shown in FIGS. 14A and 14B has a larger opening 3 as compared with that of the half-tone phase-shifting mask shown in FIGS. 13A and 13B.
FIG. 14C is a diagram of a light intensity profile obtained in using the enhancer mask of FIGS. 14A and 14B. FIG. 14D is a schematic plan view of a pattern transferred onto a wafer when the opening 3 of FIGS. 14A and 14B is a hole pattern. In this enhancer mask, just like in the half-tone phase-shifting mask, an unwanted side lobe with the local maximum value of the light intensity is caused around the opening 3 due to the influence of diffraction of the light caused at the edge of the opening 3 as shown in FIGS. 13C and 14C. The local maximum value of the light intensity of the side lobe is in proportion to the dimension of the opening 3. In particular, when the local maximum value of the light intensity of the side lobe exceeds an exposure threshold value (i.e., the minimum value of the light intensity for sensitizing a resist to be exposed) as shown in FIG. 14C, an unwanted pattern (a side lobe pattern) is disadvantageously transferred onto the periphery of a portion of the wafer corresponding to the opening 3 as shown in FIG. 14D.
As a method for preventing the occurrence of a side lobe, mask bias (modification of a mask pattern caused by proximity correction or the like) is increased to reduce the exposure amount. In this method, however, the contrast is disadvantageously lowered.
In order to overcome this problem, Patent Document 1 discloses, although which is directed to a half-tone phase-shifting mask, a method of preventing the occurrence of a side lobe in a large opening such as an accessory pattern by forming an accessory pattern or other patterns on a light-shielding region. FIG. 15 is a plan view illustrating a conventional photomask disclosed by Patent Document 1. As shown in FIG. 15, a halftone portion 2 is provided in the middle of a transparent substrate (not shown) and a light-shielding portion 5 is provided along the edge of the transparent substrate. First openings 3A each having a dimension of not more than 0.5×λ/NA are formed in the halftone portion 2 and second openings 3B each having a dimension larger than 0.5×λ/NA are formed in the light-shielding portion 5.    [Patent Document 1] Publication of Japanese Patent Application No. 9-281690